US3222417A - Catalytic dehydrogenation of hydrocarbons - Google Patents

Description

United States Patent 3,222,417 CATALYTIC DEHYDROGENATION OF HYDROCARBONS Lawrence J. Hughes, Hitchcock, Tex., assignor to Monsanto Company, St. Louis, M0., a corporation of Delaware No Drawing. Filed June 6, 1963, Ser. No. 285,877 4 Claims. (Cl. 260683.3)

The present invention relates to a process for the catalytic dehydrogenation of aliphatic hydrocarbons. More particularly, it relates to an improved process for the catalytic dehydrogenation of long-chain saturated hydrocarbons containing from 9. to 16 carbon atoms to the corresponding unsaturated hydrocarbons containing the same number of carbon atoms to the molecule.

It is well known that saturated aliphatic hydrocarbons can be converted to unsaturated hydrocarbons by catalytic dehydrogenation. A wide variety of catalytic systems and processes have been developed in the art for the dehydrogenation of n-paraflins to produce the corresponding olefins. A classic example in point is the conversion of nbutane to butenes. For the most part, the prior art processes are directed to the catalytic dehydrogenation of light, normally-gaseous parafiins, that is, C C and C hydrocarbons, employing so-called alumina-chromia type catalysts. These and other generally known catalysts, however, are not suitable for the catalytic dehyrogenation of the higher normal or straight-chain paraffins such as hexane and particularly long-chain n-paraffins, that is, nparaffins having from 9 to 16 carbon atoms. When employed with such hydrocarbons they cause formation of aromatic hydrocarbons and relatively smaller amounts of olefins. Another problem arises from that fact that longchain high boiling molecules are more easily cracked than the short-chain, low-molecular weight molecules. Under the normal dehydrogenating conditions, therefore, a major concomitant reaction in the catalytic dehydrogenation of long-chain n-parafi'ins is cracking or splitting of carbon-carbon bonds to form two or more lighter hydrocarbons one of which is usually less saturated than the other. This produces an unsaturated hydrocarbon different from that produced by simple dehydrogenation and an undesirable light parafiin. The product thus becomes a heterogeneous one from which the desired olefin can be recovered only in poor ultimate yields by a series of purification operations which can be tedious and expensive.

Accordingly, it is an object of the present invention to provide an improved process for the catalytic dehydrogenation of long-chain saturated hydrocarbons to the corresponding unsaturated hydrocarbons containing the same number of carbon atoms to the molecule which overcomes the disadvantages of the prior art.

It is a further object of the invention to provide an improved process for the catalytic dehydrogenation of normal paraffins containing from 9 to 16 carbon atoms to the corresponding monoolefins.

These and other objects and advantages of the invention which will be apparent from the following description thereof are obtained by contacting at elevated temperatures a normal parafiin containing from 9 to 16 carbon atoms in the vapor phase with a catalyst comprising copper molybdate supported on alumina or activated alumina. In the specific embodiment of the invention, the long-chain normal paraflins are converted to olefins by diluting them with steam in amounts up to 20 volumes of inert gas per volume of hydrocarbons and then contacting the mixed gases with copper molybdate supported on alumina or activated alumina at a temperature within the range from about 350 to about 650 C. and particularly from about ICC 425 to about 525 C. at contact times varying from about 0.1 to about 10 sec. It is surprising that the copper molybdate catalyst can be used to convert long-chain nparafiins to the corresponding olefins in view of the prior art teachings that the higher boiling members of the paraffinic hydrocarbons such as heptanes, hexanes, octanes and the higher members of the homologous series will tend to form unsaturated ring compounds such as aromatics when catalytically dehydrogenated.

The invention is illustrated in the following examples which, however, are not to be construed as limiting it in any manner whatsoever.

Example 1 The dehydrogenation reactor employed was a stainless steel tube approximately V2 in. in outside diameter and 3% in. long. It was immersed in a heated salt bath equipped with a thermostat for temperature control. The reactor was packed with a catalyst comprising copper molybdate supported on alumina which contained 5.5% by weight of copper oxide and 10% by weight of molybdenum trioxide, the remainder of the composition being alumina. This catalyst had been prepared as follows: A quantity of a commercially available catalyst consisting of 10% by weight of molybdenum trioxide deposited on alumina in the form of 7 tablets was impregnated by immersion of the tablets in an aqueous solution of copper nitrate. The mixture was then subjected to vacuum drawn by a water aspirator for about two hours at room temperature followed by about five minutes at a temperature of about 60 C. The vacuum was released and the mixture was tumbled at room temperature for 50 minutes. Theliquid was then decanted and the pellets were poured onto a screen and air dried. The dried pellets were charged to the reactor and activated by passing a stream of steam through them at a temperature of about 450 C. for about 2 hours.

After the catalyst had been activated, high-purity (99+%) n-dodecane was vaporized and mixed with steam in a volume ratio of two parts of steam to one of hydrocarbon. The mixture} Was preheated to about 250 C. and passed through the catalyst bed in the reactor after the temperature had been regulated and stabilized at the desired level. Flow rates were adjusted to provide the desired contact time. Pressure was maintained at essentially atmospheric throughout all the runs.

The etfiuent gas was conducted through a cold water condenser. All uncondensed gases were vented. The contents of the condenser-receiver were analyzed by means of gas chromatography. Based on the analysis, the ultimate yield of dodecene was calculated assuming 100% recovery. Results obtained in several runs are presented in Table I together with the conditions under which they were obtained.

TABLE I Temper- Contact Percent Ultimate Run No. ature, Time Dodecene Yield C. (see) in Eflluent Dodecene,

Percent K-7 452 l 4. 9 91 K-7 452 2 6. 7 93 K-8 467 1 7. 9 K-8- 468 2 10. 4 K-9 474 1 8. 4 88 K-Q. 474 2 9. 2 89 K-ll 470 2 4. 0 87 470 4 4. 4 85 Example 2 Following essentially the same procedure as in Example 1, n-dodecane was dehydrogenated in a tubular stainless steel reactor /2 in. in outside diameter and about 12 in.

long. The catalyst employed was a copper rnolybdate on alumina prepared in the same manner as that used in Example 1 but containing 2.1% by weight of copper oxide and 10% by Weight of molybdenum trioxide. Results of the runs made are recorded in Table II below together with the conditions under which they were obtained. The catalyst in Run 67 had been regenerated after use in Run 66 by heating it While a stream of air was passed through it at a temperature of about 400 C. for about 18 hours.

Using the same procedure empolyed in Example 1, ndodecane was dehydrogenated in a reactor made of Vyoor tubing about 1.2 in. in outside diameter and about 24 inches long fitted with a thermowell of /s in. stainless steel tubing. The upper half of the reactor tube was packed with a catalyst comprising copper rnolybdate supported on alumina containing 7.3% by weight of copper oxide and 10% by weight of molybdenum trioxide and prepared in the same manner as that used in Examples 1 and 2 but activated by heating to 100 C. for 1.5 hr. in a stream of nitrogen, replacing the nitrogen with steam at a rate of 1.2 g. H O per min. per liter of catalyst and heating to 450 C. for 1.5 hr. at 450 C. for 1 hr. before starting the hydrocarbon feed. The lower half of the reactor was packed with beryl saddles and functioned as a mixing and preheating section for the diluent-dodecane mixture which was introduced into the bottom of the reactor and passed upward through the two sections. Results of a series of runs conducted using various temperatures, steam-to-hydrocarbon ratios, and contact times are presented in Table III below.

TABLE III Contact Steam-to- Percent Ultimate Run No. Temp, Time Dodecane, Dodecene Yield 0. (sec.) Volume in Efiluent Dodecene,

Ratio Percent Example 4 Following essentially the same procedure as in the preceding examples, an alkane fraction or kerosene cut from which the isoparatfins had been removed consisting essentially of n-paraflins having from 11 to 14 carbon atoms was dehydrogenated in a tubular reactor 2 /8 in. in inside diameter and 36 in. long. The bed of catalyst consisting of copper rnolybdate on alumina in the form of in. x in. tablets containing about 7% copper oxide and molybdenum trioxide was about 24 in. in length. The alkane feed was diluted with steam to provide a steam-hydrocarbon volume ratio of 6:1 and was passed through the catalyst bed maintained at a temperature from about 470-480 C. at a flow rate to provide a contact time of about 12 seconds. The ultimate yield of olefins having from 11 to 14 carbon atoms based on analysis of the effluent product and assuming recovery was 91.8%.

Variations in procedure and reaction conditions from those given in the examples may be made without departing from the scope of the invention. For example, the dehydrogenation process of the invention is applicable generally to straight-chain saturated aliphatic hydrocarbons having from 9 to 16 carbon atoms such as nnonane, n-decane, n-undecane, n-tridecane, n-tetradecane, n-pentadecane and n-hexadecane. These compounds may be reacted individually or mixtures of the compounds may be employed as the starting material in the dehydrogena. tion process. If desired, the dehydrogenation may be effected in the presence of relatively inert gases as diluents. Suitable diluents in addition to the steam and nitrogen exemplified include stable hydrocarbons such as methane, hydrogen, argon, and the like. When a diluent is employed any ratio of diluent to hydrocarbon up to about 20:1 may be employed. Preferred diluent-tohydrocarbon ratios, particularly when steam is used, are those in the range from 2:1 to 10:1.

The catalyst employed herein described as copper rnolybdate is a mixture of copper oxide (CuO) and molybdenum oxide, generally molybdic trioxide (M00 supported upon alumina. Catalysts containing varying amounts of molybdenum oxide or molybdic trioxide supported on alumina are readily available commercially. These may be easily converted to the copper rnolybdate catalysts of the present invention by saturating them with a solution of a copper salt such as the sulfate, nitrate, or chloride, for example, which can then be decomposed to the oxide by heating in air, steam or an inert atmosphere at temperatures in the range from about 300 C. to about 700 C. With decomposition of the copper compound to the oxide form, there occurs simultaneously the formation of the copper rnolybdate compound by the reaction of the copper oxide and the molybdic trioxide, provided the temperatures employed are sufliciently high. Temperatures between about 450 and 650 C. are generally suitable in this step of activating the catalyst. Alternatively, the catalyst may be prepared by saturating alumina pellets with molybdic acid, then after drying the impregnated alumina, converting the acid to molybdenum trioxide by heating the impregnated alumina to a temperature of about 500 C. Thereafter, the molybdenum trioxide on alumina can be treated in the same manner as any of the available commercial catalysts referred to above.

In another method of preparation, alumina pellets are immersed in a solution of copper nitrate until they are saturated. The solution is then decanted, the pellets are dried in air, then dried at C. for from 6-12 hours followed by calcining at about 500 C. for 12 hr. The resulting copper oxide on alumina is then impregnated with a molybdenum salt such as ammonium rnolybdate by immersing it in a solution thereof to the point of saturation. The excess solution is drained oflf, the pellets are dried in air, dried at 120 C. for 6 hours, and then calcined at 500 C. for 12 hours to yield the copper molybdate on alumina. Other methods may also be employed. For example, a mixture of copper nitrate and ammonium rnolybdate may be maintained in solution by adjusting the pH of the solution with ammonium hydroxide. (In an unadjusted solution cobalt rnolybdate is precipitated.) The alumina is then immersed in this solution and saturated with it. Excess solution is decanted as in the other methods, the impregnated alumina is air dried followed by drying at 120 C. for 6 hours and then calcination at 500 C. for about 12 hours, Or, copper rnolybdate may be coprecipitated with alumina followed by drying the gel formed, grinding, forming into pellets and calcining at 500 C.

Generally, for the purposes of the present invention the copper molybdate contains from about 0.7% to about 14% of copper oxide and from about 3% to about 12% of molybdic trioxide with the remainder of the catalyst composition being alumina. Preferred catalyst compositions contain from about 1.5 to about 7% by weight of copper oxide and from about 5% to about by weight of molybdic trioxide on alumina. Preferably, gamma-alumina is employed as the catalyst support; however, other activated aluminas can also be employed. The catalyst may be used in any suitable solid form such as powder, granules, tablets, and the like.

With continued use, the catalyst may become deactivated by deposition of carbonaceous material thereon. When this occurs, it can be readily reactivated or regenerated by burning off the deposits in a stream of oxygen or air suitably diluted for temperature control with an inert gas or steam.

The vapors of the long-chain paraflin hydrocarbon to be dehydrogenated can be contacted with the catalyst at any temperature within the range from 350 to 650 C. Preferably, temperatures from about 425 to about 525 C. are employed depending upon the particular hydrocarbon feed material. Ordinarily, atmospheric pressure is used but the process can be conducted advantageously under reduced pressures. Superatmospheric pressures, while they can be used, are usually to be avoided because at high pressures the equilibrium is shifted to favor the paraffin.

Contact time is not critical in that times of contact and temperature are interchangeable over a fairly wide range and vary likewise with the particular catalyst composition employed. Generally, contact times employed may vary from about 0.1 to about 10 seconds. Preferred contact times of 1 to 5 seconds are used in conjunction with the preferred temperatures.

Any of a number of materials can be employed for the construction of the reactor. Essentially the same results are obtained with reactor tubes fabricated from copper, glass, and stainless steels, for example.

What is claimed is:

1. An improved process for the dehydrogenation of long-chain n-paraffins to produce olefins having a corresponding number of carbon atoms in the molecule which comprises contacting n-parafiins containing from 9 to 16 carbon atoms in the vapor phase with a catalyst consisting essentially of copper molybdate supported on alumina at temperatures from about 350 C. to about 650 C. and at contact times from about 0.1 to about 10 seconds.

2. An improved process for the dehydrogenation of long-chain n-parafiins to produce olefins having a corresponding number of carbon atoms in the molecule which comprises contacting n-paraflins containing from 9 to 16 carbon atoms in the vapor phase and diluted with a relatively inert gas in volume proportions up to 20 parts of inert gas per part of paraflin with a catalyst consisting essentially of copper molybdate supported on alumina, said catalyst containing from about 0.7% to about 14% by weight of copper oxide and from about 3% to about 12% by weight of molybdic trioxide, at temperatures from about 350 C. to about 650 C. and at contact times from about 0.1 to about 10 seconds.

3. An improved process for the dehydrogenation of lon chain n-parafiins to produce olefins having a corresponding number of carbon atoms in the molecule which comprises contacting n-paraifins containing from 9 to 16 carbon atoms in the vapor phase and diluted with an inert gas to provide a diluent-to-hydrocarbon volume ratio from about 2:1 to about 10:1 with a catalyst consisting essentially of copper molybdate supported on alumina, said catalyst containing from about 0.7% to about 14% by weight of copper oxide and from about 3% to about 12% by weight of molybdic trioxide at temperatures from about 420 C. to about 525 C. and at contact times from about 0.1 to about 10 seconds.

4. An improved process for the dehydrogenation of long-chain n-parafiins to produce olefins having a corresponding number of carbon atoms in the molecule which comprises contacting n-parafiins containing from 9 to 16 carbon atoms in the vapor phase and diluted with an inert gas to provide a diluent-to-hydrocarbon volume ratio from about 2:1 to about 10:1 with a catalyst consisting essentially of copper molybdate supported on alumina, said catalyst containing from about 1.5% to about 7% by weight of copper oxide and from about 5% to about 10% by weight of molybdic trioxide at temperatures from about 425 C. to about 525 C. and at contact times from about 1 to 5 seconds.

References Cited by the Examiner UNITED STATES PATENTS 1,986,241 1/1935 Wulff et al 260-168 2,932,673 4/ 1960 Melik et al 260-6833 2,985,596 5/ 1961 Pitzer 260-6833 3,050,469 8/1962 Morgan et a1 260-6833 3,076,858 2/ 1963 Frevel et al 260-677 3,148,228 9/ 1964- Franz et al 260-6833 3,161,696 12/1964 Eder et al 260-6833 PAUL M. COUGHLAN, Primary Examiner.

ALPHONSO D. SULLIVAN, Examiner.

Claims (1)

1. AN IMPROVED PROCESS FOR THE DEHYDROGENATION OF LONG-CHAIN N-PARAFFINS TO PRODUCE OLEFINS HAVING A CORRESPONDING NUMBER OF CARBON ATOMS IN THE MOLECULE WHICH COMPRISES CONTACTING N-PARAFFIS CONTAINING FROM 9 TO 16 CARBON ATOMS IN THE VAPOR PHASE WITH A CATALYST CONSISTING ESSENTIALLY OF COPPER MOLYBDATE SUPPORTED ON ALUMINA AT TEMPERATURES FROM ABOUT 350*C. TO ABOUT 650*C. AND AT CONTACT TIMES FROM ABOUT 0.1 TO ABOUT 10 SECONDS.